SCSI command descriptor block parsing state machine

ABSTRACT

A hard disk controller integrated circuit of a SCSI target device comprises a dedicated command descriptor block &#34;CDB&#34; parsing state machine. The dedicated CDB parsing state machine parses incoming six-byte, ten-byte and twelve-byte SCSI command descriptor blocks and writes information from corresponding fields in the six-byte, ten-byte and twelve-byte SCSI command descriptor blocks into predetermined memory locations. In some embodiments, there are sixteen such predetermined memory locations which together comprise a 16×8 register file. A microprocessor coupled to the hard disk controller integrated circuit is therefore relieved of the burden of parsing incoming SCSI command descriptor blocks.

CROSS REFERENCE TO MICROFICHE APPENDICES

Microfiche Appendix A, which is a part of the present disclosure,comprises 1 sheet of microfiche having a total of 89 frames. MicroficheAppendix A is a specification for a SCSI interface portion of a diskcontroller integrated circuit.

Microfiche Appendix B, which is a part of the present disclosure,comprises 1 sheet of microfiche having a total of 83 frames. MicroficheAppendix B is a specification for a SCSI sequencer block disposed withina SCSI interface portion of a disk controller integrated circuit. Partof the SCSI sequencer block is described in VHDL hardware descriptionlanguage source code and part is described by schematics. MicroficheAppendix B also includes schematics generated from the VHDL source codeby logic synthesis software.

Microfiche Appendix C, which is a part of the present disclosure,comprises 1 sheet of microfiche having a total of 45 frames. MicroficheAppendix C is a specification for SCSI command descriptor block parsinghardware disposed within a SCSI interface block of a disk controllerintegrated circuit. Part of the SCSI command descriptor block parsinghardware is described in VHDL hardware description language source codeand part is described by schematics. Microfiche Appendix C also includesschematics generated from the VHDL source code by logic synthesissoftware.

Microfiche Appendix D, which is a part of the present disclosure,comprises 1 sheet of microfiche having a total of 8 frames. MicroficheAppendix D is VHDL hardware description language source code of anembodiment of a "receive ID tag instruction state machine" and of anembodiment of a "send ID tag instruction state machine".

A portion of the disclosure of this patent document contains materialwhich is subject to copyright protection. The copyright owner has noobjection to the facsimile reproduction by anyone of the patent documentor the patent disclosure, as it appears in the Patent and TrademarkOffice patent files or records, but otherwise reserves all copyrightrights.

FIELD OF THE INVENTION

This invention relates to SCSI bus interfaces. More particularly, theinvention relates to parsing of SCSI command descriptor blocks byhardware on a hard disk controller integrated circuit.

BACKGROUND INFORMATION

The Small Computer System Interface (SCSI or commonly called the "SCSIbus") is a popular device independent parallel bus. The SCSI bus can,for example, be used to removably connect multiple devices includinghard disk drives, printers and other input/output peripheral devices toa host computer. For background, the reader is referred to "Fast TrackTo SCSI", Integrated Circuits Division of Fujitsu Microelectronics,Inc., published by Prentice Hall (1991), the SCSI-1 specification, andthe SCSI-2 specification (documents X3.131 and X3T9.2 of the AmericanNational Standards Institute).

FIG. 1 (PRIOR ART) is a simplified block diagram illustrating onepossible SCSI bus configuration. Both an initiator device 1 as well as atarget device 2 are shown coupled to a SCSI bus 3. SCSI bus 3 comprisesnine data conductors (eight for data and one for parity), nine controlconductors, and other power and ground conductors. Devices 1 and 2 arecoupled in parallel to the bus conductors, corresponding SCSI terminalsof each device being coupled to the same corresponding bus conductor.Typically each conductor is resistively coupled to a voltage of aninactive state. To "assert" a signal onto a conductor, a device mustdrive to conductor to a voltage of an active state against the resistivecoupling of the conductor. If not driven, a conductor will return to itsinactive state.

Each device connected to a SCSI bus is classified as either an initiatoror as a target. Initiator devices cause target devices on the bus toperform commands whereas target devices perform commands for theinitiators. There can be multiple initiators and multiple targets on aSCSI bus.

Target device 2 in FIG. 1 is a hard disk computer peripheral devicecomprising a hard disk controller integrated circuit 4, a buffer memory5, a first microprocessor 6, a second microprocessor 7, a hard disk 8,head electronics and actuator 9, and read channel electronics 10. If,for example, initiator 1 were to attempt to write data to disk 8, thenthe initiator 1 would output a write command onto the SCSI bus 3. TheSCSI bus is initially in a "bus free phase" in which the SCSI bus isidle. To initiate the write command, initiator 1 asserts a BSY signalonto an OR-tied BSY control conductor of SCSI bus 3, thereby causing thebus to enter an "arbitration bus phase". During the arbitration busphase, each initiator arbitrates for the bus with the other initiatorsby asserting the appropriate one of the data conductors of the SCSI buscorresponding with a SCSI identifier (SCSI ID) unique to the initiator.Because each SCSI ID has an assigned priority, the initiator with thehighest priority, in this case initiator 1, determines that it has thehighest priority by detecting the other data conductors. The highestpriority initiator, here initiator 1, asserts a select signal SEL onto aSEL conductor of the bus to indicate to other initiators and targetsthat the bus is busy.

After winning control of the bus through arbitration, initiator 1selects the target device of interest, in this case target 2, in a"selection bus phase". Initiator 1 asserts its SCSI ID as well as theSCSI ID of the target onto the data conductors of the SCSI bus. When thetarget detects its SCSI ID on the data conductors, the target respondsby asserting the BSY signal onto the OR-tied BSY conductor. The busfree, arbitration, and selection bus phases are control phases.

With initiator 1 now in control of SCSI bus 3, and with target 2identified as the target, target 2 requests a SCSI command frominitiator 1 in a "command bus phase". The SCSI bus has a C/D signal andan associated C/D conductor for distinguishing control and data on thebus. The C/D signal being asserted indicates control information isbeing passed over the bus whereas the C/D signal being deassertedindicates data being passed over the bus. The SCSI bus also has an I/Osignal and an associated I/O conductor for indicating the direction offlow of information across the bus. The I/O signal being assertedindicates information flow from target to initiator whereas the I/Osignal being deasserted indicates information flow from initiator totarget. Accordingly, the target asserts the C/D signal indicatingcontrol information and deasserts the I/O signal indicating informationflow from initiator to target. Initiator 1 then responds by sending thecommand over the bus to target 2 byte by byte using two controlconductors of the SCSI bus, the REQ and ACK conductors, for handshaking.Each SCSI command, called a SCSI command descriptor block (CDB),consists of multiple bytes, either six, ten or twelve bytes. The commandcontains information which includes a SCSI operation code indicating thetype of command to be performed.

FIG. 2A (PRIOR ART) is a diagram illustrating the fields in the SCSIoperation code byte of a SCSI command CDB. The first byte, byte 0, ofall SCSI commands is a SCSI operation code. A group code of 0 indicatesthat the command is a six-byte command; a group code of 1 or 2 indicatesthat the command is a ten-byte command; and a group code of 5 indicatesthat the command is a twelve-byte command.

FIGS. 2B, 2C and 2D (PRIOR ART) are diagrams illustrating six-byte,ten-byte, and twelve-byte commands, respectively. The transfer lengthfield, if required by the command specified by the command code of theSCSI operation code, specifies the number of blocks (or bytes)transferred upon execution of the command. Whether the transfer lengthinformation of the transfer length field is in bytes or blocks isdetermined by the type of command. If the command is a write command,the command descriptor block includes a logical block address (LBA) ofthe first block to be transferred as well as the number of blocks to betransferred during execution of the write command.

Hard disk controller integrated circuit 4 comprises a SCSI interfaceportion 11 for interfacing with the SCSI bus 3, a disk controllerportion 12 for interfacing with the hard disk 8, and a buffer managerportion 13 for controlling a flow of data through buffer memory 5between the SCSI interface portion 11 and disk controller portion 12. Itis the SCSI interface portion 11 which receives commands from the SCSIbus 3.

For a write command, the read/write head of disk 8 must usually be movedin a seek operation to an appropriate location on disk where the data isto be written. Accordingly, microprocessor 7 is instructed to move thehead to the correct location by microprocessor 6. While the seekoperation is being carried out, data can be received from the initiator1 over SCSI bus 3 for later writing to disk 8. SCSI interface portion 11therefore configures the buffer manager portion 13 to store incomingdata into buffer memory 5.

When the target 2 is ready to receive data, the target 2 deasserts theI/O signal and deasserts the C/D signal thereby causing the bus to entera "data out phase". Initiator 1 then sends byte after byte of data totarget 2 over the SCSI bus using the REQ and ACK signals forhandshaking. Successive bytes of data are placed in buffer memory 5.When the seek operation is complete, the disk controller portion 12begins writing data received from the buffer memory 5 to disk 8.

After, for example, all the data of the write command has been receivedinto buffer memory 5, the target 2 may cause the bus to enter a "statusbus phase" by asserting the C/D and I/O signals and by deasserting asignal MSG on an associated control conductor MSG. The target 2 thensends to initiator 1 a status byte indicating whether or not the commandwas executed without error.

If, for example, the write of information to disk 8 was successful, thentarget 2 may cause the bus to enter a "message in phase" by assertingthe C/D, I/O and MSG signals. With the bus in the message in phase,target 2 may send initiator 1 a SCSI defined message. If, for example, acommand complete message is sent, then initiator 1 will be able toexamine the status byte after completion of the command. Target 2releases SCSI bus 3 by releasing the BSY signal whereupon the SCSI busreenters the bus free phase. Whereas the bus free, arbitration andselection phases are called control phases, the command, data, statusand message phases are called information transfer phases.

In order to reduce the cost of a target SCSI hard disk drive peripheral,the first and second microprocessors 6 and 7 may be combined into asingle microprocessor. Such a single microprocessor is, however,frequently interrupted to control SCSI bus operations. The response ofthe microprocessor to other tasks may therefore be undesirably slow. If,for example, a SCSI write command is to be sent to the target, then themicroprocessor may be interrupted after the initiator 1 has selected thetarget. This interrupt allows the microprocessor to set up facilities inthe SCSI interface to receive the command bytes. After the command byteshave been received into the SCSI interface (for example into afirst-in-first-out memory), the microprocessor is again interrupted.This allows the microprocessor to interpret the meaning of the commandbytes.

Because the same type of information may be contained in different onesof the bytes of six-byte, ten-byte and twelve-byte SCSI commanddescriptor blocks, the microprocessor may read the various bytes of thecommand descriptor block out of a first-in-first-out memory into whichthe bytes were initially written, determine what type of information iscontained in each byte dependent upon whether the command descriptorblock is a six-byte, ten-byte or twelve-byte command descriptor block,and then write selected bytes of the information into predeterminedmemory locations accessible by the microprocessor for future use.

If the command is determined to be a write command, then themicroprocessor would configure the rest of the target including thebuffer manager to accommodate the data to be transferred duringsubsequent execution of the command. Then after the data transfer iscomplete, the microprocessor is again interrupted by the hard diskcontroller integrated circuit to indicate that it is time to send thestatus byte and the command complete message byte to initiator 1. Themicroprocessor then can load the status byte and command completemessage byte into a FIFO and enable an operation to shift to FIFO out,thereby placing the status byte and command complete message bytes ontothe SCSI bus. After the placing of status and command complete messagebytes onto the SCSI bus, the microprocessor is again interruptedalerting the microprocessor that the operation is completed.

Not only is the microprocessor burdened by the need to determine thelocations of different types of information in different types ofcommand descriptor blocks and the need to handle numerous interrupts inthe execution of a command, but the microprocessor may also beinterrupted in the event that more data is written to the disk driveperipheral than the disk drive peripheral has room to accommodate in itsbuffer memory. If, for example, the buffer memory 5 is full whenadditional data is to be received from the bus, then the microprocessorwill be interrupted by the hard disk controller integrated circuit. Thisallows the microprocessor the opportunity to instruct the SCSI interfaceto disconnect itself from the SCSI bus so that the bus can be used totransfer information between other devices. Then, after the disk driveperipheral again has the ability to receive additional data from theinitiator, the microprocessor is again interrupted so that themicroprocessor can instruct the SCSI interface portion 11 to againarbitrate for the bus and to reselect the bus in a SCSI "reselection busphase". Additional interrupts may therefore be involved in disconnectingfrom the bus and then reconnecting to the bus. Moreover, suchdisconnecting and reconnecting can occur multiple times when executing asingle SCSI command leading to still more microprocessor interrupts. Ahard disk controller integrated circuit is therefore desired whichreduces microprocessor overhead in parsing command descriptor blocks andwhich reduces the number of microprocessor interrupts in a SCSI bustarget when executing a SCSI command.

SUMMARY

An SCSI command descriptor block parsing state machine in accordancewith an embodiment of the present invention writes information fromcorresponding field six-byte, ten-byte and twelve-byte SCSI commanddescriptor blocks into the same register of a set of registers. Forexample, bytes 4, 8 and 9 contain the low byte of the transfer lengthfield of six-byte, ten-byte and twelve-byte command descriptor blocks,respectively, yet a command descriptor block parsing state machine inaccordance with an embodiment of the present invention places the lowbyte of the transfer length field into the same single-byte registerregardless of whether the command descriptor block is a six-byte,ten-byte or twelve-byte command descriptor block.

In some embodiments, a hard disk controller integrated circuit comprisesa sequencer and a command descriptor block parsing state machine. Thecommand descriptor block parsing state machine is a dedicated statemachine which parses information from the various fields of incomingSCSI command descriptor blocks into predetermined correspondingregisters in the set of registers. The sequencer then executes SCSIcommands or parts of SCSI commands based on the contents of the set ofregisters as loaded by the command descriptor block parsing statemachine. A microprocessor coupled to the hard disk controller integratedcircuit can access the set of registers. In some embodiments, the set ofregisters is also operable as a first-in-first-out memory therebyenabling the microprocessor to load the contents of the registers withSCSI message bytes and to output those message bytes serially onto theSCSI bus.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 (PRIOR ART) is a simplified diagram of an initiator device and atarget device coupled to a SCSI bus.

FIGS. 2A-2D (PRIOR ART) are diagrams illustrating formats of six-byte,ten-byte and twelve-byte SCSI command descriptor blocks.

FIG. 3 is a simplified diagram of an initiator device and a targetdevice coupled to a SCSI bus in accordance with an embodiment of thepresent invention.

FIG. 4 is a simplified block diagram of a SCSI interface portion of ahard disk controller integrated circuit in accordance with an embodimentof the present invention.

FIGS. 5A and 5B are diagrams illustrating registers of the SCSIinterface portion of FIG. 4.

FIG. 6 is a diagram illustrating sequencer instructions located insequencer memory.

FIGS. 7A-7D are flow diagrams illustrating operation of a sequencer inaccordance with an embodiment of the present invention.

FIG. 8 is a state diagram illustrating operation of an embodiment of thesequencer of the SCSI sequencer block.

FIGS. 9A-9J are state diagrams illustrating operation of the "s₋ ins"instruction state machine of the sequencer of FIG. 8.

FIG. 10A (PRIOR ART) is a diagram illustrating the fields of a six-byteSCSI command descriptor block and FIG. 10B is a diagram showing whereinformation from each of the fields is written into the sixteen-byteCFIFO.

FIG. 11A (PRIOR ART) is a diagram illustrating the fields of a ten-byteSCSI command descriptor block and FIG. 11B is a diagram showing whereinformation from each of the fields is written into the sixteen-byteCFIFO.

FIG. 12A (PRIOR ART) is a diagram illustrating the fields of atwelve-byte SCSI command descriptor block and FIG. 12B is a diagramillustrating where information from each of the fields is written intothe sixteen-byte CFIFO.

FIGS. 13A-13D are flowcharts illustrating a operation of a "CDB parsingstate machine" in accordance with an embodiment of the presentinvention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 3 is a simplified block diagram illustrating an embodiment inaccordance with the present invention. An initiator device 201 as wellas a target device 202 are coupled to a SCSI bus 203. Target device 202comprises a hard disk controller integrated circuit 204, a buffer memory205, a microprocessor 206, a hard disk 208, head electronics andactuator 209, and read channel electronics 210. Hard disk controllerintegrated circuit 204 comprises a disk controller portion 212, a buffermanager portion 213, a SCSI interface portion 211, and a microprocessorinterface portion 214. Each of the portions 211, 212 and 213 has a localmicroprocessor interface block 211A, 212A and 213A, respectively, forcommunication with the microprocessor interface portion 214. Althoughthe hard disk controller integrated circuit 204 is illustrated directlycoupled to the SCSI bus 203, circuitry such as external bus drivers maybe disposed between the hard disk controller integrated circuit 204 andthe SCSI bus 203 in some embodiments.

FIG. 4 is a simplified block diagram of SCSI interface portion 211 ofhard disk controller integrated circuit 204. SCSI interface portion 211comprises local microprocessor interface and SCSI registers block 211A,a parity generation and check block 215, a data first-in-first-outmemory (DFIFO) 216, a control first-in-first-out memory (CFIFO) 217, afirst-in-first-out memory control block 218, a host transfer controlblock 219, a SCSI signal and phase control block 220, a sequencer block221, an arbitration/selection state machines block 222, and a hosttransfer counter block 223. The signal conductors 224 designated MICROin FIG. 4 couple the SCSI interface portion 211 to microprocessor 206 ofthe target device 202 via the microprocessor interface portion 214. Thesignal conductors 225 labelled BUFFER in FIG. 4 couple the SCSIinterface portion 211 to buffer manager portion 213 of the diskcontroller integrated circuit 204. The signal conductors 226 designatedSCSI in FIG. 4 couple the SCSI interface portion 211 to SCSI bus 203.

Local microprocessor interface and SCSI registers block 211A interfacesto all other blocks of the SCSI interface portion 211. The connectionsare not shown in FIG. 4 so as not to over-crowd the diagram.Microprocessor 206 can read or write numerous registers located in thelocal microprocessor interface block 211A via the microprocessorinterface portion 214 of the disk controller integrated circuit 204.

DFIFO block 216 is a 16×18 first-in-first-out memory used to transferdata between SCSI bus 203 and buffer 205 as controlled by the buffermanager portion of the disk controller integrated circuit. The interfacebetween DFIFO 216 and SCSI bus 203 is 9-bits wide for a narrow SCSIinterface and is 18-bits wide for a wide SCSI interface.

CFIFO block 217 is a 16×8 register file used for transferring controlbytes (command, status and/or message) to and from SCSI bus 203. It isorganized as sixteen 8-bit registers. Each of the registers can be reador written independently of the others by microprocessor 206 via theHCFIFO0-HCFIFOF registers (21h-30h). When transferring bytes to the SCSIbus, the CFIFO usually functions as a normal FIFO with a FIFO count anda data pointer. The microprocessor 206 can access the FIFO count and thedata pointer via registers HCFCNT (1Fh) and HCFPTR (20h), respectively.

FIFO control block 218 controls the CFIFO and DFIFO blocks byinterfacing with the buffer manager portion 213 and by keeping track ofthe status of the two FIFOs. Host transfer control block 219 controlsdata transfer between SCSI bus 203 and the buffer manager portion 213 bycontrolling data transfer between SCSI bus 203 and the CFIFO block 217.The host transfer control block 219 handles the REQ/ACK handshaking ofthe SCSI protocol and controls transfer stops when there is inadequateroom in buffer memory 205 for data of the command being executed. Hosttransfer counter 223 counts blocks of information transferred betweenSCSI bus 203 and buffer memory 205. Parity generation and check block215 checks the parity of data received from SCSI bus 203 with the paritybit of the SCSI bus 203 and also generates parity bits when data is sentfrom CFIFO block 217 to the SCSI bus 203. During transfer from buffermemory 205 to SCSI bus 203, parity is received from the buffer memory205 and is passed through the DFIFO 216 to the SCSI bus without beingregenerated. SCSI signal and phase control block 220 handlesmiscellaneous control signals for interfacing to the SCSI bus.Arbitration and selection state machines block 222 controls the SCSIsignal sequences for performing SCSI arbitration, selection andreselection as well as for responding to selection and reselection byother SCSI devices on the SCSI bus.

SCSI sequencer block 221 comprises a sequencer, a sequencer memory, an"instruction state machine", a "CDB parsing state machine", a "receiveID tag instruction state machine", and a "send ID tag instruction statemachine". The sequencer and its associated state machines togetherexecute SCSI sequences of sequencer instructions in target modeincluding automatically receiving an autoread or autowrite command fromthe SCSI bus and proceeding to transfer data without waiting for anycommunication from microprocessor 206.

FIGS. 5A-5B illustrate "registers" of the SCSI interface portion 211.Although some of the registers may actually involve counters or otherdigital circuitry, the counters or other digital circuitry are referredto as "registers" here because they can be accessed by microprocessor206. An R indicates that the corresponding register can be read bymicroprocessor 206. A W indicates that the corresponding register can bewritten by microprocessor 206. A R/W indicates that the register canboth be read and written by microprocessor 206. The address at whicheach of the registers can be accessed by microprocessor 206 is alsogiven. Individual bits in the registers are given descriptive labels.

Microprocessor 206 starts the sequencer sequencing through a series ofinstructions located in sequencer memory by initializing a SCSIsequencer address register HSADR (01h) with the address of the firstinstruction of the instruction sequence and then setting a sequencer runS₋ RUN bit in a register HSEQEN (02h). After the S₋ RUN bit has beenset, the sequencer proceeds through sequencer memory from instruction toinstruction until a stop address STOP (2Fh) is reached. Some sequencerinstructions have branch conditions. If the branch condition of asequencer instruction is true, then the sequencer will branch to theinstruction address specified in the branch address field of theinstruction. The sequencer can therefore be made to stop by branching tothe stop address. When the sequencer stops, the sequencer done bitSEQDONE of sequencer status register HSEQSTAT (03h) is set and thesequencer address register HSADR contains the address of the lastinstruction executed before reaching the STOP address.

FIG. 6 is a diagram illustrating sequencer instructions located insequencer memory. Locations 00h-0Fh contain single-instruction sequenceswhich allow a single instruction to be executed. Locations 00h-00Fh arelocations, the contents of which are user-definable but are fixed attime of manufacture of the hard disk controller integrated circuit.After the instruction located at the starting address is executed, thesequencer stops. Locations 10h-2Eh, on the other hand, containmulti-instruction sequences. Locations 10h-2Eh are ROM locations thecontents of which are fixed as illustrated in FIG. 6. Location 2Fh,another ROM location, is the STOP address to which multi-instructionsequences may branch to stop. Locations 30h-3Fh are RAM locations, thecontents of which can be written by microprocessor 206. Accordingly,user-defined sequences can be written into the RAM locations of thesequencer memory by the microprocessor 206 and then executed later bythe sequencer.

Table 1 below sets forth a summary explanation of each of the sequenceroperation codes of the instructions executable by the sequencer.Appendices A and B are to be consulted for additional details.

                                      TABLE 1    __________________________________________________________________________    VALUE         LABEL   DESCRIPTION    __________________________________________________________________________    0    NOP     No operation    1    RCV.sub.-- IDTAG                 Receive Identify and Queue Tag: If attention is                 asserted (ATNILAT=1 of register HSTAT2), switch                 to "message out bus phase" and receive a byte                 from the SCSI bus into the CFIFOO register. If                 the byte is a valid Identify message byte, set                 the identify received status bit (IDEN of                 register HCTL1). If attention is still asserted,                 disconnection is enabled, and receiving of the                 queue tag message is enabled (ENQTAG=1 in                 register HSTAT0). Two queue tage message bytes                 are received into the CFIFO1 and CFIFO2                 registers. If there is an outstanding unqueued                 command (Ix.sub.-- OC=1 and Ix.sub.-- QACT=0), receiving of                 the                 queue tag message is disabled. If there are one                 or more outstanding queued commands (Ix.sub.-- OC=1 and                 Ix.sub.-- QACT=1) and attention is negated after the                 Identify byte, the instruction is halted.    2    RCV.sub.-- CMD                 Receive Command: Switch to "command bus phase".                 Receive the first byte of the command from the                 SCSI bus into the CFIFO3 register. Check the                 group code in bits 7:5 of that byte. If the                 group code is 0, 1, 2, or 5, then receive the                 remaining command bytes into the CFIFO, and the                 instruction is done. For any other group code,                 set the unexpected data status bit (UNEXDATA=1 in                 register HERRSTAT) and halt the sequencer.    3    XFR.sub.-- DATA                 Transfer Data: If the host transfer counter                 (registers HXCT0,1) has not expired, and if a                 write command (WROP=1 in register HCTL0), then                 switch to "data out bus phase", else switch to                 "data in bus phase". Enable buffer DMA transfer                 and wait for buffer DMA Done (bit BDMA DONE in                 register HXFRSTAT), then wait until BWAIT.sub.-- H=0                 from the buffer manager, and the instruction is                 done if BUFERR H=0 from the buffer manager.    4    SND.sub.-- CHK                 Send Check Condition Status: Load the check                 condition status byte (02h) into the host data                 out register 0 (18h), switch to "status bus                 phase", send the message byte to the SCSI bus in                 register PIO mode, instruction is done.    5    RCV.sub.-- MSG                 Receive Message: If attention is negated, the                 instruction is done. If attention is asserted,                 switch to "message out bus phase" and receive                 message bytes froom the SCSI bus into CFIFO until                 attention is negated, at which time the                 instruction is done. If CFIFO becomes full and                 attention is still on, set unexpected attention                 status (UNEXATN bit of register HERRSTAT) and                 halt the sequencer.    6    SND.sub.-- MSG                 Send Message: Switch to "message in bus phase",                 flush out the bytes in the CFIFO to the SCSI bus,                 instruction is done.    7    SND.sub.-- SDP                 Send Save Data Pointer Message: Load SDP message                 byte (02h) into host data out register 0 (HDOR0),                 switch to "message in bus phase", send the                 message byte to the SCSI bus in register PIO                 mode, instruction is done.    8    SND.sub.-- DIS                 Send Disconnect Message: Load disconnect message                 byte (04h) into host data out register 0 (HDOR0),                 switch to "message in bus phase", send the                 message byte to the SCSI bus in register PIO                 mode, go bus free, instruction is done.    9    SND.sub.-- STS                 Send Status: Switch to "status bus phase", send                 one byte ot the SCSI bus from the CFIFO,                 instruction is done.    A    SND.sub.-- IDTAG                 Send Identify and Queue Tag: If the active                 identify received status bit (bit ACT.sub.-- IDEN in                 register HACT.sub.-- IDQ) is reset, then instruction is                 done. Else, switch to "message in bus phase",                 load a value of 80h+ACT.sub.-- UN (i.e. identify                 message with the logical unit number for the                 active command) into register HDOR0, and send the                 message bye to the SCSI bus in register PIO                 mode. If active queue tag received status bit                 (bit ACT.sub.-- QTAG in register HACT.sub.-- IDQ) is reset,                 then instruction is done. Else, load a value of                 20h (i.e. first queue tag byte) into register                 HDOR0, and send the byte to the SCSI bus, then                 the load the contents of register HACT.sub.-- QT2                 (second queue tag byte) into HDOR0 and send one                 more byte to the SCSI bus, instruction is done.    B    SND.sub.-- GS                 Send Good Status: Load good status byte (00h)                 into host data out register 0 (HDOR0), switch to                 "status bus phase", send the message byte to the                 SCSI bus in register PIO mode, instruction is                 done.    C    SND.sub.-- CC                 Send Command Complete Message: Lod CC message                 byte (00h) into host data out register 0 (HDOR0),                 switch to "message in bus phase", send the                 message byte to the SCSI bus in register PIO                 mode, go bus free, instructio is done.    D    RSELO   Reselect Out: Attempt to reselect the initiator                 specified in the DESTID 3:0! bits in the HIDOUT                 register. The instruction is done when the hard                 disk controller integrated circuit is connected                 to the SCSI bus in Target mode, i.e. either the                 reselection completed or the hard disk controller                 integrated circuit was selected by an initiator.    E    RSELONTHR                 Reselect Out On Threshold: Attempt to reselect                 the initiator specified in the DESTID 3:0! bits                 in the HIDOUT register when the buffer threshold                 signal from the buffer manager portion (BTHR.sub.-- H)                 is asserted. The instruction is done when the                 hard disk controller integrated circuit is                 connected to the SCSI bus in target mode, i.e.                 either the reselection completed or the hard disk                 controller integrated circuit was selected by an                 initiator.    F    reserved    __________________________________________________________________________

Table 1

Table 2 below sets forth a summary explanation of each of the branchconditions which may be present in an instruction executable by thesequencer. Many of the branch conditions of Table 2 are dependent uponthe state of one or more register bits located in the SCSI interfaceblock.

                                      TABLE 2    __________________________________________________________________________    VALUE         LABEL     DESCRIPTION    __________________________________________________________________________    0    NB        No branch.    1    BRCH      Unconditional Branch    2    B.sub.-- CFLOCK.sub.-- S                   Branch if CFIFO access is locked (bit                   CFLOCK S=1 in register HCTL1).    3    B.sub.-- QFULL                   Branch if queue is full (bit QFULL=1 in                   register HSTAT1).    4    B.sub.-- XIP                   Branch if a data transfer is in progress (bit                   XIP=1 in register HCTL1).    5    B.sub.-- AUTO                   Branch if current command is an autowrite or an                   ESP command (bit AWR=1 or bit ESP=1 in register                   HSTAT0).    6    B.sub.-- NDISCXFR                   Branch if disconnection on data transfer is not                   allowed (DP=0 in register HCTL1 or DISCXFR=0 in                   register HCTL0).    7    B.sub.-- NNORMDISC                   Branch if disconnection is not allowed (DP=0 in                   register HCTL1), or if both of the following                   conditions are false:                   a.                     The current command is a normal read                     (NORMRD=1 in register HSTAT0) and                     disconnect on normal read is allowed                     (DISCNRD=1 in register HCTL0).                   b.                     The current command is a normal write                     (NORMWR=1 iin register HSTAT0) and                     disconnect on normal write is allowed                     (DISCNWR=1 in register HCTL0).    8    B.sub.-- XCONT                   Branch if transfer continue is enabled (XCONT=1                   in register HCTL0).    9    B.sub.-- XDONE                   Branch if data transfer is done (XDONE=1 in                   register HSTAT1).    A    B.sub.-- CCE                   Branch if sending of command complete is                   enabled (WROP=1 in register HCTL1 and ENCCWR=1                   in register HCTL2, or WROP=0 and ENCCRD=1 in                   register HCTL2).    B    B.sub.-- NDISCST                   Branch if disconnect on status is disabled                   (DISCST=0 in register HCTL0) or disconnection                   is not allowed (DP=0 in register HCTL1).    C    B.sub.-- SELIN                   Branch if a select has occurred (SELIN=1 in                   registesr HSTAT2).    D    B.sub.-- TAGNOK                   Branch if queue tag is not OK (TAGNOK=1).    E    B.sub.-- GPB0                   Branch if general purpose branch 0 bit (GPB0 in                   register HCTL2) is set.    F    B.sub.-- GPB1                   Branch if general purpose branch 1 bit (GPB1 in                   register HCTL2) is set.    __________________________________________________________________________

Table 2

Operation of the sequencer block 221 of the SCSI interface portion ofthe disk drive integrated circuit is explained by reference to anexecution of an autowrite command. FIGS. 7A-7D are a flow chartillustrating sequencer instruction execution by the sequencer. In theflowchart of FIGS. 7A-7D, a rectangular box with a rounded cornersindicates a sequencer operation code of a sequencer instruction whereasa following box with pointed corners indicates a branch condition of thesequencer instruction. A sequencer instruction may be represented onlyas one box with rounded corners where the sequencer instruction has nobranch condition. Conversely, a sequencer instruction may be representedonly as one box having pointed corners where the sequencer operationcode is a NOP (no operation) but where the sequencer instruction doeshave a branch condition.

As indicated by the label IDCMD in FIG. 7A, instruction flow begins bymicroprocessor 206 loading register HSADR with the address 10h and bysetting the S₋ RUN bit in register HSEQEN to start the sequencer. Thebranch condition of the first sequencer instruction executed results inthe sequencer of sequencer block 221 waiting until CFIFO block 217 is nolonger locked. If CFIFO block 217 is unlocked as determined by bitCFLOCK₋ S of register HCTL1 being a "0", then process flow proceeds tolocation 11h. A state machine in the arbitration/selection statemachines block 222 interacts with the SCSI signal and phase controlblock 220 to respond to initiator 201 for selection. When target 202 isselected by initiator 201, the state machine within thearbitration/selection state machines block 220 loads bits OTHERID3through OTHERID0 of register HOTHERID (14h) with the SCSI ID of theinitiator that made the selection. As indicated in FIG. 7A, thesequencer will halt if instruction execution reaches this locationbefore target 202 is connected to the SCSI bus. The sequencer resumesinstruction execution after a target mode connection to the SCSI bus isestablished (i.e. after target 202 is selected by initiator 201).

Next, the receive identify and queue tag instruction at location 11h isexecuted by the sequencer. The sequencer initiates operation of the"instruction state machine" which in turn initiates operation of the"receive ID tag instruction state machine" of sequencer block 221. Thisstate machine causes an identify message byte containing a logical unitnumber to be loaded into CFIFO0 of CFIFO block 217. If the commandcontains the optional two-byte queue tag message of the SCSI protocol,such queue tag bytes are loaded into CFIFOa and CFIFO2 of CFIFO 217block and the QTAG2 bit of register HSTAT1 is set after loading of thesecond queue tag byte. The attention signal ATN on conductor ATN of theSCSI is controlled by the initiator to indicate to the target whetherqueue tag bytes are coming or whether the first byte of the commanddescriptor block is coming. As indicated in FIG. 7A, if the queue isfull, a branch to STOP address 2Fh is performed, otherwise instructionflow proceeds to location 12h. Queuing is explained in more detailbelow.

Location 12h contains the receive command instruction. The sequencerinitiates operation of the "instruction state machine" which in turninitiates operation of the "CDB parsing state machine" of the sequencerblock 221. The CDB parsing state machine parses the command, loadsinformation from different fields of the command into correspondingother CFIFO locations and then generates an interrupt to themicroprocessor to notify the microprocessor 206 that a new command hasbeen received and parsed. If a data transfer is already in progress,instruction flow proceeds to location 16h as illustrated, otherwiseinstruction flow proceeds to location 13h.

The interrupt is generated onto conductor 206A (see FIG. 3) as follows.Status register HSEQSTAT (03h) contains a CDBDONE bit. The CDBDONE bitis set by the "CDB parsing state machine" when the command has beenparsed. Each bit of the HSEQSTAT status register has an associated maskbit in interrupt enable register HSEQINTEN (04h). If any unmasked bit inHSEQSTAT is set, then an interrupt condition will be signalled tomicroprocessor interface block 214 via interrupt conductor HINT₋ L0.Microprocessor interface block 214 uses this interrupt signal from thelocal microprocessor interface block 211A to generate an interrupt tomicroprocessor 206 on interrupt conductor 206A. Accordingly, the settingof an unmasked status bit in the HSEQSTAT register results in amicroprocessor interrupt.

At location 13h, execution of the NOP with the B₋ AUTO branch conditionresults in the sequencer determining whether the command in the CFIFOrequires microprocessor intervention or whether the command in the CFIFOshould be executed without microprocessor intervention. In order for theSCSI interface portion 211 to determine that it should carry out thecommand without microprocessor intervention, the command must be eitheran autowrite command as indicated by an autowrite bit AWR in registerHSTAT0 being set or an ESP command as indicated by an ESP bit inregister HSTAT0 being set. In the presently described scenario, thecommand is an autowrite command. Three conditions must be met in orderfor the sequencer to automatically branch to the data transfer phase inthe presently described scenario.

First, an enable autowrite (ENAWR) bit of register HCTL0 (0Bh) must beset indicating that autowrite commands are enabled. This bit can bewritten by microprocessor 206 beforehand to enable or disable the SCSIinterface portion 211 from performing autowrites.

Second, a bit NORMWR of register HSTAT0 (0Ch) must be set indicatingthat the command is a predetermined type of write command, for example,a "normal write command." There are no "normal write commands" in thetwelve-byte command format. Accordingly, if the command receivedcomprises twelve bytes, the command is not a normal write. For asix-byte command, a normal write command is a write command for whichthe control byte (byte five of FIG. 2B) is all zeros. The setting ofbits in the control byte may indicate that numerous commands are to belinked together, thereby precluding the command from being classified anormal write. For a ten-byte command, a normal write command is a writecommand for which the control byte (byte nine of FIG. 2C) is all zerosas in the case of a six-byte command, but where the two reserved fieldsof bytes one and six also contain all zeros. A bit in one of thesereserved fields being set may, for example, indicate a special writecommand such as a disable on caching write command. A command with anonzero bit in the reserved fields is therefore determined not to be anormal write. Accordingly, bit NORMWR of register HSTAT0 (0Ch) will havebeen set if the decode of the command bytes in the CFIFO indicates anormal write, if a valid identify message byte was received, and if thementioned command and message bytes were received for an initiatorhaving a valid initiator SCSI ID.

Third, the bit ESP in register HSTAT0 (0Ch) must be zero. If all threeconditions are met (if NORMWR=1, if ENAWR=1 and if ESP=0), then anautowrite bit AWR in register HSTAT0 is set.

In the presently described scenario, the command is an autowritecommand, thereby resulting in bit AWR being set and the branch conditionof the NOP at location 13h being true. Because the branch condition istrue, a plurality of registers are loaded as follows. As indicated inFIGS. 2B-2D, the received command contains a field called the "transferlength field". This field contains a number indicating the number ofblocks (or bytes) of data to be transferred by execution of the command.Upon the branch condition of location 13h being true, a counter in thehost transfer counter block 223 is loaded with transfer lengthinformation contained in the transfer length field of the receivedcommand. This counter is called a register (specifically registerHXCTR0,1) because it can be read and written by microprocessor 206.Furthermore, a register called the active identify and queue tagregister (register HACT₋ IDQ) is loaded with the SCSI ID of theinitiator which issued the command. A register called the active queuetag byte 2 register (register HACT₋ QT2) is loaded with queue taginformation from any queue tags received by the target. The initiatoridentification information and the queue tag information is supplied tothe registers HACT₋ IDQ and HACT₋ QT2 from CFIFO block 217. Four bitsDESTID3 through DESTID0 of the register HIDOUT (13h) are also loadedwith the SCSI ID of the destination which was previously stored in fourbits OTHERID3 through OTHERID0 of register HOTHERID (14h). A counter(denoted register HLBACTR0,1) is loaded with the least significantsixteen bits of the logical block address of the first block of data tothe transferred. The logical block address of the first block of data tobe transferred is, for example, indicated by bytes 2-5 of a ten-bytecommand. Counter HLBACTR0,1 is called a register because it can be readand written by the microprocessor. During the transfer of data,HLBACTR0,1 is incremented upon the transfer of each block (or byte) ofdata so that HLBACTR0,1 contains the least significant sixteen bits ofthe logic block address of the present data block. A bit WROP in hostcontrol register HCTL1 is also loaded with a "1" if the command is awrite command and is loaded with a "0" if the command is a read command.The registers and bits HACT.sub. - IDQ, HACT₋ QT2, DESTID3-0, HLBACTR0,1and WROP are all located in the microprocessor interface and SCSIregisters block 214.

After the loading of these registers and bits, sequencer executionautomatically proceeds to the data transfer sequence (XFR sequence) ofFIG. 7B without waiting for a microprocessor communication. The transferdata instruction located at location 19h is therefore executed. Assuccessive blocks of data are transferred from SCSI bus 203 to thebuffer manager portion 213, the host transfer counter HXCTR0,1 disposedin block 223 is decremented and the logical block address in HLBACTR0,1in block 214 is incremented.

If, for example, the buffer memory 205 cannot accommodate all the dataof the autowrite command as fast as the data can be supplied from theSCSI bus 203, then the buffer manager portion 213 signals that no roomexists in the buffer by asserting a BNOROOM₋ H signal (see FIG. 4) tothe host transfer control block 214 of the SCSI interface portion. Asindicated in FIG. 7B, process flow proceeds to location 1Ah where thesequencer checks to determine if microprocessor 206 has allowed thetarget to disconnect from the SCSI bus. If the target is allowed todisconnect, then the sequencer executes the instructions at locations1Bh and 1Ch to cause two messages required by the SCSI protocol prior toa disconnect to be output onto the SCSI bus 203. The "save data pointer"SCSI message is output onto the SCSI bus by execution of the instructionat location 1Bh and the "disconnect" SCSI message is output onto theSCSI bus by execution of the instruction at location 1Ch. The sequencerthen activates the SCSI signal and phase control block 220 to place SCSIbus 203 in the bus free state. With target 202 now disconnected fromSCSI bus 203, the SCSI bus 203 is now available for other data transfersto occur, for example between another initiator and another target.

FIG. 7C illustrates a later determination of whether to reconnect toSCSI bus 203 at location 1Dh. Based on a threshold condition asindicated by signal BTHR₋ H (see FIG. 4) received from the buffermanager portion 213, a determination is made by the sequencer as towhether to initiate reconnection to the SCSI bus. If a determination ismade to reconnect, the SCSI ID of the initiator must be sent out ontothe SCSI bus in order to allow the initiator of the interrupted commandto be reconnected with the target. Accordingly, the contents of bitsDESTID3 through DESTID0 which were previously stored in register HIDOUTon the autowrite branch are output onto the SCSI bus. When the initiatorrecognizes its SCSI ID on the SCSI bus and responds by selecting thetarget, the instruction at location 1Eh is executed to send the SCSIidentify and queue tag information of the interrupted command which werepreviously stored in the HACT₋ IDQ and HACT₋ QT2 registers back to theinitiator (A "sequencer state machine" initiates operation of an"instruction state machine" which in turn initiates operation of a "sendID tag instruction state machine" which actually sends the SCSI identifyand queue tag information). Process control then proceeds back to thedata transfer sequence of FIG. 7B.

If after the data transfer is completed, the sequencer determines thatthe SCSI interface portion 211 is enabled to complete the command, thenthe branch instruction of location 1Fh causes process flow to proceed tothe good status (GS) sequence of FIG. 7D. The determination of whetherthe SCSI interface portion 211 is enabled to complete the command ismade with reference to the B CCE branch condition of Table 2.

The instruction at location 24h facilitates a test for a valid queuetag. In SCSI-2 it is possible for an initiator to send many commands(each having a queue ID tag) to a target in a burst of commands withoutwaiting for each command to be executed. For example, an initiator maysend a first read command, followed by a second read command, followedby a third read command. If the data to be read by the first command islocated on an outer track of a disk, the data to be read by the secondcommand is located on an inner track of the disk and the data to be readby the third command is located on a center track of the disk, thetarget may reorder the commands first command-third command-secondcommand for execution so that the head of the disk seeks from the outertrack, to the center track, to the inner track, thereby speeding theseek operations and reducing disk drive response time. Multiple commandsare therefore "queued" in the target provided that no two outstandingcommands received from a single initiator have the same queue ID tag. Ifa received command for a given initiator is determined to have a uniquequeue tag with respect to all outstanding queue tags for that initiator,then the received command is said to have been validated.

Accordingly, after CFIFO 217 is initially loaded with the queue taginformation for the received command, microprocessor 206 receives theCDBDONE interrupt and then reads the queue tag. Meanwhile, the sequencerhas proceeded to enter the data transfer phase without waiting. At thispoint the SCSI ID of the initiator that selected the target is presentin register HOTHERID (14h).

To enable tag checking, there are three sets (A, B and C) of three bitsstored in the registers HIAB₋ STAT (3Ah) and HIC₋ STAT (3Bh), one setfor each of three initiators. The SCSI ID of the three initiators A, Band C are available in bits 3 through 0 in registers at locations 34h,36h and 38h. The SCSI ID of the initiator of the received command iscompared with the SCSI Ids of the three initiators A, B and C. If thereis a match, then the queue tag of the recently received command may beinvalid and is checked.

If an outstanding command exists for the matching initiator, then theoutstanding command bit OC (in register 3Ah or 3Bh) for the matchinginitiator will be set. If another command is then received for aninitiator the OC bit of which is set, then the TAGNOK bit (in register3Ah or 3Bh) for that initiator bit is set. The value of the TAGNOK bitof the relevant initiator is also the value of a global TAGNOK bit inregister HSTAT1 (0Eh). Once a TAGNOK bit is set, the TAGNOK bit must bereset by microprocessor 206, thereby providing an interlock mechanismpreventing the sequencer from sending status and command complete bytesprematurely until the microprocessor has had a chance to validate thequeue tag. If the microprocessor 206 determines that the receivedcommand has a valid queue tag, then the microprocessor resets the TAGNOKbit for the appropriate initiator so that the global TAGNOK bit inregister HSTAT1 is reset to a "0". Otherwise, the global TAGNOK bitremains a "1".

In SCSI-2, commands without queue tags cannot be queued. The QACT bit ofeach of the three sets of bits in registers HIAB₋ STAT and HIC₋ STATindicates that the SCSI ID of the received command matches the SCSI IDof the corresponding initiator but that the received command does nothave a queue tag.

If when the sequencer reaches location 24h (see FIG. 7D) the globalTAGNOK bit of register HSTAT1 (0Eh) is cleared in the present scenarioindicating that the queue tag was validated by the microprocessor orthere is only one outstanding command from this initiator and thereforethe tag is valid, then the send good status instruction is executed bythe sequencer at location 25h. (The send good status instruction is arelatively simple operation and therefore is carried out by the"instruction state machine".) After a send command complete message isoutput onto the SCSI bus by execution of the instruction at location27h, the SCSI signal and phase control block 220 is activated to causethe SCSI bus to go into the bus free phase and the command completeoccurred bit CCOCR is set in register HSEQSTAT (03h) of themicroprocessor interface and SCSI registers block 211A in order togenerate an interrupt signal onto conductor HINT₋ L0. As a result,microprocessor 206 is alerted via microprocessor interface portion 214that the sequencer has completed execution of the SCSI command.

If in another scenario the microprocessor has not yet cleared the TAGNOKbit when the sequencer reaches location 24h, then the sequencer willdisconnect from the SCSI bus. When disconnected, the SCSI bus can beused by other devices on the bus. The microprocessor can then later setup the SCSI interface portion to reconnect to the SCSI bus and send thestatus byte and the command complete message byte.

Not only does the SCSI interface portion 211 of the disk controllerintegrated circuit 204 proceed from the command bus phase to the datatransfer bus phase without waiting for a communication from themicroprocessor for commands determined to be autowrite commands, but theSCSI interface portion 211 also proceeds from the command bus phase tothe data transfer bus phase without waiting for a communication from themicroprocessor for commands called "ESP commands". Note that executionof the B₋ AUTO command at location 13h in FIG. 7A also causes thesequence to proceed to the data transfer sequencer (denoted XFR sequencein FIG. 7A) if the ESP bit of register HSTAT0 (OCh) is set. In a commanddetermined to be an ESP command, the logical block address of thecommand matches an expected logical block address which was previouslyloaded by microprocessor 206 into four registers HESPLBA0, HESPLBA1,HESPLBA2 and HESPLBA3 (3Ch-3Fh).

For purposes of the specific embodiment, the term "autotransfer"encompasses both autowrite commands and autoread commands. The ordinaryautowrite command described above is one type of autowrite command andthe ESP read command is one type of autoread command. The ESP writecommand is a specialized type of autowrite command.

Assume that initiator 201 is reading a sequence of blocks of data atsequential logical block addresses from target 202 using a correspondingsequence of SCSI read commands. If it were possible for the target 202to determine that the next read command is the next of the sequence ofread commands, then the appropriate data could be loaded into buffermemory 205 and the buffer manager portion 213 could be set up to providethat data before the read command is received. As a result, responsetime of the target 202 to the anticipated read command would bedecreased when the anticipated read command is actually received becausethe buffer manager portion 213 would be configured and ready beforehand.

In accordance with an embodiment of the present invention,microprocessor 206 determines that the next command from a giveninitiator is likely to be a command having a given logical blockaddress. Microprocessor 206 loads the anticipated logical block addressinto the four registers HESPLBA0 (3Ch), HESPLBA1 (3Dh), HESPLBA2 (3CEh)and HESPLBA3 (3Fh) so that the buffer manager is configured for data atthe anticipated logical block address. The received command is thentested to determine if it is an ESP command. If the command isdetermined to be an ESP command, then the ESP bit in register HSTAT0(0Ch) is set thereby enabling the sequencer to branch to the datatransfer sequence at location 13h in the flow of FIG. 7A.

The current command is determined to be an ESP command and the ESP bitis set if: 1) ESP commands are enabled as determined by bit ENESP beingset in register HCTL0 (0Bh); 2) the logical block address of thereceived command in registers CFIFO5 through CFIFO8 matches theanticipated logical block address in registers HESPLBA3 throughHESPLBA0; and 3) the command is either a normal write command when ESPwrites are enabled by bit ESPRD in register HCTL0 being cleared, or thecommand is a normal read command when ESP reads are enabled by bit ESPRDin register HCTL0 being set. Microprocessor 206 can therefore enableeither ESP read branching or ESP write branching by either setting orclearing the ENESP and ESPRD bits. Normal write commands and normal readcommands are indicated by bits NORMWR and NORMRD being set in registerHSTAT0 (0Ch), respectively.

Assume now that the initiator were to attempt to write ten blocks ofdata into ten sequential logical block addresses. The initiator maychoose to do this with one autowrite command having a transfer length often and a starting logical block address of X. Alternatively, theinitiator may choose to accomplish this write using two autowritecommands, one having a transfer length of five and a logical blockaddress X, the other having a transfer length of five and a logicalblock address of X+5. If autowrite and ESP are both enabled, the SCSIinterface portion 211 is able to distinguish between writes ofcontiguous data and writes of non-contiguous data. If the second writecommand is determined to be an ESP command, then the data of the secondwrite command can be written into buffer memory 205 starting at alocation which is contiguous with the locations of previously writtendata of the first write command. If, on the other hand, the second writecommand is determined not to be an ESP command, then the data of thesecond write command can be written into other locations in the buffermemory 205 which are not contiguous with previously written data of thefirst write command. Distinguishing such contiguous data andnon-contiguous data aids in buffer management because all of suchcontiguous data may be later written at once from buffer memory to onearea of disk 208 without intermittent seeking. Storing all thiscontiguous data into buffer memory 205 at contiguous locations thereforesimplifies the subsequent reading of buffer memory 205 by the buffermanagement portion 213.

In accordance with the illustration in FIG. 4, a host autowrite pulsesignal is output by SCSI interface portion 211 to buffer manager portion213 on conductor HAWRP₋ H if the command received is an ordinaryautowrite. Similarly, a host ESP pulse signal is output by SCSIinterface portion 211 to buffer manager portion 213 on conductor HESPP₋H if the command received is an ESP command. Which of the two conductorsHAWRP₋ H and HESPP₋ H carries the pulse determines whether the data iscontiguous data or non-contiguous data. Conductor HWRITE₋ H indicates tothe buffer manager portion 213 whether the command is a read or a write.The signals allow the buffer manager portion 213 to prepare forcontiguous or non-contiguous reads or writes of data.

As mentioned above, the SCSI sequencer block 221 comprises six basicparts: 1) a sequencer, 2) a sequencer memory, 3) an "instruction statemachine", 4) a "CDB parsing state machine", 5) a "receive ID taginstruction state machine", and 6) a "send ID tag instruction statemachine". The "instruction state machine" is a state machine forcarrying out simple sequencer operation codes and is denoted "s₋ ins" inthe VHDL code of microfiche Appendix B. The sequencer in turncomprises: 1) interface circuitry for reading and writing sequencermemory by microprocessor 206 (the interface circuitry is denoted "s₋srctl" in the schematics of microfiche Appendix B), 2) circuitry whichselects the next instruction to be executed by the sequencer (thiscircuitry is denoted "s₋ seqsel" in the VHDL code of microfiche AppendixB), and 3) a sequencer state machine which is denoted "s₋ seq" in theVHDL code of microfiche Appendix B. It is to be understood, however,that all embodiments in accordance with the present invention need notbe partitioned in the particular way described and that the specificpartitioning presented here is merely illustrative of one possibleembodiment of a sequencer block.

In operation of the specific embodiment, the "s₋ seq", "s₋ seqsel" andthe "sequencer memory" operate together so that the sequencer statemachine steps through the instructions in the sequencer memory bybranching to the appropriate locations. After reaching a new sequencermemory location, the s₋ seq sequencer state machine signals the s₋ ins"instruction state machine" to handle the operation indicated by thesequencer operation code at the new location. If the operation indicatedis a relatively simple operation, the s₋ ins "instruction state machine"carries out the operation itself. If, on the other hand, the operationindicated is relatively complex, the s₋ ins "instruction state machine"initiates one of the other state machines (the "CDB parsing statemachine", the "receive ID tag instruction state machine" or the "send IDtag instruction state machine") to carry out the operation. After theoperation of the current sequencer operation code has been carried out,the "instruction state machine" s₋ ins has fulfilled its function andtherefore signals the sequencer state machine s₋ seq to continue. Thesequencer state machine then determines which location in sequencermemory to proceed to next.

FIG. 8 is a state diagram illustrating operation of one possiblesequencer in accordance with the present invention. After reading thesequencer instruction, the sequencer outputs an instruction go INSGOsignal to the s₋ ins "instruction state machine". The state "EXEC₋ INS"in the lower right of FIG. 8 represents the execution of the instructionby the s₋ ins "instruction state machine". After the s₋ ins "instructionstate machine" has completed the operation indicated by the sequenceroperation code, the s₋ ins "instruction state machine" outputs aninstruction done INSDONE signal to the sequencer so that the sequencercan continue to its next state.

FIGS. 9A-9J are state diagrams illustrating operation of the s₋ ins"instruction state machine". If, for example, the sequencer instructionindicated by the sequencer operation code is the receive commandinstruction (RCV₋ CMD), then the s₋ ins "instruction state machine"operates as indicated in FIG. 9B. The state "INS₋ XFR" in the lowerright of FIG. 9B represents the operation of the "CDB parsing statemachine". The s₋ ins "instruction state machine" starts the "CDB parsingstate machine" by outputting the CDB parsing state machine go CDBSMGOsignal to the "CDB parsing state machine". After the "CDB parsing statemachine" has parsed the SCSI command descriptor block, the "CDB parsingstate machine" outputs a CDB state machine done CDBSMDONED signal to thes₋ ins "instruction state machine" so that the "instruction statemachine" can continue to its next state.

If, on the other hand, the sequencer instruction indicated by thesequencer operation code is the receive ID tag command instruction (RCV₋IDTAG), then the s₋ ins "instruction state machine" operates asindicated in FIG. 9A. The state "INS₋ XFR" in the lower right of FIG. 9Arepresents the operation of the "receive ID tag instruction statemachine". The s₋ ins "instruction state machine" starts the "receive IDtag instruction state machine" by outputting the receive ID taginstruction state machine go RCVIDSMMGO signal to the "receive ID taginstruction state machine". After the "receive ID tag instruction statemachine" has competed its function, the "receive ID tag instructionstate machine" outputs a receive ID tag instruction state machine doneRCVIDSMDONED signal to the s₋ ins "instruction state machine" so thatthe "instruction state machine" can continue to its next state.

If, however, the sequencer instruction indicated by the sequenceroperation code is the send ID tag command instruction (SND₋ IDTAG), thenthe s₋ ins "instruction state machine" operates as indicated in FIG. 9H.The state "INS₋ XFR" in the lower right of FIG. 9H represents theoperation of the "send ID tag instruction state machine". The s₋ ins"instruction state machine" starts the "send ID tag instruction statemachine" by outputting the send ID tag instruction state machine goSNDIDSMMGO signal to the "send ID tag instruction state machine". Afterthe "send ID tag instruction state machine" has competed its function,the "send ID tag instruction state machine" outputs a send ID taginstruction state machine done SNDIDSMDONED signal to the s₋ ins"instruction state machine" so that the "instruction state machine" cancontinue to its next state.

The receive and send ID tag instruction state machines (both describedin the same file "S₋ idqtag.vhd") are described further in VHDL hardwaredescription language source code in microfiche Appendix D. The CDBparsing state machine ("S₋ cdb.xhd") is described further in VHDLhardware description language source code in microfiche Appendix C.

COMMAND DESCRIPTOR BLOCK PARSING

As illustrated in FIG. 7A, the SCSI command descriptor block of acommand is parsed by execution of the sequencer instruction RCV₋ CMD atlocation 12h. Three portions of hardware are primarily responsible forcarrying out the parsing of command descriptor blocks: 1) the "CDBparsing state machine", denoted "s₋ cdb" in the VHDL code of Appendix C,located in the SCSI sequencer block 221, 2) a CFIFO control block,denoted "s₋ cfctl" in the schematics of Appendix C, located in the FIFOcontrol block 218, and 3) CFIFO block 217 denoted "s₋ cfifo" in theschematics of Appendix C.

The CFIFO block 217 is a 16×8 array of registers. In a specificembodiment, each register comprises a plurality of transparent D-latchescoupled in parallel. Microprocessor 206 can read and write the registersof the CFIFO block (21h-30h) one at a time in any sequence, except thatwriting by the microprocessor is disabled when signal CFLOCK₋ M₋ H isasserted. Except for microprocessor reads and writes, the CFIFO isaccessed as a first-in-first-out memory using the four-bit CFIFO pointer(bits CFPTR3 through CFPTR0 in register HCFPTR (20h)) which selects oneof the sixteen registers for access.

The FIFO control block 218 in FIG. 4 comprises two portions: a block forcontrolling DFIFO block 216, and the CFIFO control block "s₋ cfctl" ofinterest here. The CFIFO control block "s₋ cfctl" comprises logic thatgenerates control signals for reading and writing the CFIFO as well astwo counter/registers: the CFIFO pointer CFPTR (bits 3 through 0 ofregister 20h), and the CFIFO count register CFCNT (bits 4 through 0 ofregister 1Fh). The CFIFO count register is a five-bit up/down counterwhose value indicates the number of bytes of information which ispresent in the CFIFO.

The CDB parsing state machine "s₋ cdb" (also called the CDB statemachine) is the state machine described above in connection with theexecution of a SCSI autowrite command. This state machine is a dedicatedstate machine in contrast with an instruction based sequencer or aprocessor. The "CDB parsing state machine" controls which CFIFO registercan be written with information from the SCSI bus 203. The CDB parsingstate machine controls the CFIFO via the CFIFO pointer and by supplyingwrite signals to the CFIFO to cause the CFIFO register indicated by thepointer to be loaded with information from SCSI bus 203. After receivingthe first byte (byte 0) of a SCSI command into CFIFO byte 0 (CFIFO0 at30h), the "CDB parsing state machine" determines from the group codefield (bits 5-7 of byte 0) whether the CDB being received is a six-byte,ten-byte or twelve-byte CDB. Depending on which type of CDB is beingreceived, the "CDB parsing state machine" writes the information fromeach of the fields of the CDB into a corresponding predeterminedlocation in the sixteen-byte CFIFO (21h-30h).

FIG. 10A (PRIOR ART) is a diagram illustrating the fields of a six-bytecommand descriptor block and FIG. 10B is a diagram showing whereinformation from each of the fields is written into the sixteen-byteCFIFO of the present invention. FIG. 11A (PRIOR ART) is a diagramillustrating the fields of a ten-byte command descriptor block and FIG.11B is a diagram showing where information from each of the fields iswritten into the sixteen byte CFIFO of the present invention. FIG. 12A(PRIOR ART) is a diagram illustrating the fields of a twelve-bytecommand descriptor block and FIG. 12B is a diagram showing whereinformation from each of the fields is written into the sixteen byteCFIFO of the present invention. Accordingly, it is seen that informationfrom corresponding fields of six-byte, ten-byte and twelve-byte commanddescriptor blocks are written into the same location in the CFIFO. Forexample, the low byte of the transfer length field (denoted XFRLEN 7:0!in FIGS. 10A, 11A and 12A) is received as byte 4 of a six-byte CDB, byte8 of a ten-byte CDB, and byte 9 of a twelve-byte CDB. The informationcontent of the this low byte is, however, always written by the "CDBparsing state machine" into the same CFIFO register, register CFIFOC.Information from the other fields is similarly parsed and written intoother predetermined registers of the CFIFO as indicated in FIGS. 10-12.If the command received does not contain a particular field of thesixteen-byte CFIFO, then the locations of the CFIFO which otherwisewould hold the missing information are written with zeros in accordancewith an embodiment of the present invention.

FIG. 13A-13D is a flowchart illustrating an operation of a "CDB parsingstate machine" in accordance with an embodiment of the presentinvention. Note that labels starting with "H" in FIG. 5B correspond withsimilar labels in FIGS. 10-13 which lack the "H" but are otherwiseidentical. For example, labels HCFIFO0 through HCFIFOF of FIG. 5B referto the same registers as labels CFIFO0 through CFIFOF in FIGS. 10-13,respectively. Refer to Appendix C for additional details with respect tothe specific embodiment. The "instruction state machine" starts the "CDBparsing state machine" with the CDB state machine go signal CDBSMGO. The"CDB parsing state machine" signals the "instruction state machine" thatthe CDB has been parsed with the CDB state machine done signalCDBSMDONED. The "D" at the end of a signal name indicates that thecorresponding signal without the "D" has been delayed by one clockcycle. Note, for example, that the first bubble of FIG. 13A referencessignal CDBSMGOD whereas the arrow going into the lower right bubble inFIG. 9B references signal CDBSMGO. It is to be understood, however, thatother means for mapping the fields of FIGS. 10A, 11A and 12A into oneset of memory locations such as that of FIG. 10B may be realized andthat the present invention is not limited to the specific state machinedescribed.

Although specific embodiments of the present invention have beendescribed for instructional purposes in order to illustrate the presentinvention, the present invention is not limited thereto. The definitionof an autotransfer command for purposes of determining whether toexecute a command without waiting for a communication from amicroprocessor can be different from the specific definition used by thesequencer of the specific embodiment described. In some embodiments,some bits in the reserved fields of a command may be set and the commandmay nevertheless be considered an autotransfer command. Moreover, insome embodiments, a microprocessor may be interrupted more than twotimes for the execution of an autotransfer command. Although thespecific embodiment of the CDB parsing state machine deparses fields ofinformation into a set of transparent D-latches, other informationstorage devices may be used. The information storage devices in someembodiments are a plurality of microprocessor accessible registers whichtogether comprise a first-in-first-out memory. In some embodiments, theSCSI interface portion is modular and can be realized on a singleintegrated circuit along with various other circuits. The processor may,for example, be integrated onto the same chip as the disk controller. Inother embodiments, a disk controller integrated circuit operates inconjunction with two microprocessors external to the disk controllerintegrated circuit. Accordingly, various adaptations, modifications andsubstitutions of various of the features of the specific embodimentsdescribed can be practiced without departing from the scope of theinvention as defined in the appended claims.

I claim:
 1. A method, comprising the steps of:using a dedicated statemachine of a disk controller integrated circuit to parse six-byte,ten-byte and twelve-byte SCSI command descriptor blocks and to storeparsed information from said SCSI command descriptor blocks into saiddisk controller integrated circuit; and generating a signal in said diskcontroller integrated circuit indicative of whether said parsing andstoring is completed.
 2. The method of claim 1, wherein said six-byteSCSI command descriptor block contains a single transfer length byte,wherein said ten-byte SCSI command descriptor block contains a leastsignificant transfer length byte and a most significant transfer lengthbyte, wherein said twelve-byte SCSI command descriptor block contains aleast significant transfer length byte, a most significant transferlength byte, and two additional transfer length bytes, whereininformation from said single transfer length byte of said six-byte SCSIcommand descriptor block, information from said least significanttransfer length byte of said ten-byte SCSI command descriptor block, andinformation from said least significant transfer length byte of saidtwelve-byte SCSI command descriptor block are all mapped into a singlebyte of memory in said disk controller integrated circuit.
 3. The methodof claim 1, wherein said disk controller integrated circuit furthercomprises a sequencer, said method further comprising the step of:usingsaid signal to cause said sequencer to transition states after saidparsing and storing is completed.
 4. A SCSI disk controller integratedcircuit, comprising:a first information storage device; a secondinformation storage device; a third information storage device; and adedicated state machine coupled to said first, second and thirdinformation storage devices, said dedicated state machine writingcommand code information and transfer length information from a six-byteSCSI command descriptor block into in said first and second informationstorage devices, respectively, said dedicated state machine writingcommand code information from a ten-byte SCSI command descriptor blockinto in said first information storage device, said dedicated statemachine writing transfer length information from said ten-byte SCSIcommand descriptor block into in said second and third informationstorage devices.
 5. The SCSI disk controller integrated circuit of claim4, wherein said transfer length information of said six-byte SCSIcommand descriptor block consists of eight bits, said transfer lengthinformation of said ten-byte SCSI command descriptor block consists ofsixteen bits, said second information storage device consists of eightbits, and said third information storage device consists of eight bits.6. The SCSI disk controller integrated circuit of claim 5, wherein saidfirst, second and third information storage devices are registers. 7.The SCSI disk controller integrated circuit of claim 6, wherein saidregisters are individually accessible registers of a memory of saidintegrated circuit.
 8. The SCSI disk controller integrated circuit ofclaim 7, wherein said registers are operable as a first-in-first-outmemory.
 9. The SCSI disk controller integrated circuit of claim 4,wherein a SCSI command descriptor block comprises plurality of bytes,said dedicated state machine determining which of said plurality ofbytes to write into said second information storage device based oninformation in said first byte of said plurality of bytes.
 10. The SCSIdisk controller integrated circuit of claim 9, wherein said informationin said first byte is information in a command descriptor block groupcode.
 11. A SCSI disk controller integrated circuit, comprising:aplurality of memory locations; means for parsing six-byte and ten-byteSCSI command descriptor blocks and for storing information from aplurality of fields of said SCSI command descriptor blocks into saidmemory locations such that information from corresponding fields in saidsix-byte and ten-byte SCSI command descriptor blocks are written intothe same ones of said memory locations; and a sequencer which reads saidmemory locations, said sequencer being coupled to said means forparsing.
 12. The SCSI disk controller integrated circuit of claim 11,wherein an eight bit transfer length field of said six-byte SCSI commanddescriptor block is stored by said means for parsing into a first eightof said memory locations, wherein a first eight bits of a sixteen bittransfer length field of said ten-byte SCSI command descriptor block isstored by said means for parsing into said first eight memory locations,and wherein a second eight bits of said sixteen bit transfer lengthfield of said ten-byte SCSI command descriptor block is stored by saidmeans for parsing into a second eight of said memory locations.
 13. TheSCSI disk controller integrated circuit of claim 11, wherein saidsequencer signals said means for parsing to start parsing a SCSI commanddescriptor block and wherein said means for parsing signals saidsequencer that said SCSI command descriptor block is parsed.